A fiber laser system is used in the fields of processing machines, medical instruments, measurement apparatuses, and the like. The fiber laser system emits light amplified by an amplification optical fiber. In some cases, in the fiber laser system, signal light amplified by the amplification optical fiber constructed with a double-clad fiber is emitted from a core of the amplification optical fiber. The amplified signal light is incident on a core of a delivery fiber constructed with a single-clad fiber connected to the amplification optical fiber. Next, the light is allowed to propagate through the delivery fiber to a desired site to be emitted.
However, the light emitted from the amplification optical fiber may include surplus pumping light emitted from the clad besides the signal light emitted from the core. As described above, in the case in which the delivery fiber is connected to the amplification optical fiber, the surplus pumping light is incident on the clad of the delivery fiber. In addition, in some cases, due to mismatch in mode field diameter between the signal light propagating through the core of the amplification optical fiber and the signal light propagating through the core of the delivery fiber or due to deviation in axis, deviation in angle, or the like in the connection of the amplification optical fiber to the delivery fiber, a portion of the signal light emitted from the amplification optical fiber may be incident on the clad of the delivery fiber.
The light incident on the clad of the delivery fiber is absorbed by the cover layer of the delivery fiber, so that a problem that the cover layer is burned out may occur. Therefore, it is preferable that the light incident on the clad of the delivery fiber is appropriately emitted.
Patent Document 1 listed below discloses a fusion splicing structure of an optical fiber emitting light incident on a clad. In the fusion splicing structure, a double-clad fiber and a single-clad fiber are connected to each other in a linear shape, and at least a fusion point of the single-clad fiber is covered with a block having high thermal conductivity. Therefore, even in the case in which light is incident from the double-clad fiber on the clad of the single-clad fiber and heat is released due to the emission of the light, the heat is absorbed by the block, so that deterioration of the cover of the optical fiber can be prevented.
In addition, Patent Document 2 listed below discloses a laser delivery apparatus emitting light incident on a clad. In the laser delivery apparatus, a portion of the optical fiber in which a portion of the clad is exposed is bent with a predetermined radius of curvature and is covered with a light transmitting member, and a light absorbing member and a cooling member are installed in the light transmitting member. In this configuration, since the clad has an exposed portion, light propagating through the clad can be substantially completely removed.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-310277    [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-002608